1. Field of the Invention
The present invention relates to piezoelectric transformer inverters in which loads are driven by piezoelectric transformers. More specifically, the invention relates to piezoelectric transformer inverters capable of being used for lighting discharge tubes such as cold-cathode tubes and for adjusting the luminance of the discharge tubes.
2. Description of the Related Art
Recently, backlight liquid crystal display devices have been widely used. As a light source for backlight, a fluorescent tube, such as a cold-cathode fluorescent tube, is used.
In order for a fluorescent tube to be lit, a high-tension AC voltage must be applied. In addition, as an input power source for a portable information-processing device such as a notebook personal computer, a battery is usually used in conjunction with an AC adapter. Thus, in order to light the above-mentioned fluorescent tube, it is necessary to convert a low DC voltage into a high AC voltage capable of lighting the fluorescent tube. As a conventional fluorescent-tube lighting system, a DC/AC inverter incorporating an electromagnetic transformer has been used.
However, in order to improve miniaturization, efficiency, and operational stability of equipment, a piezoelectric transformer inverter incorporating a piezoelectric transformer has recently been used as an alternative to an electromagnetic transformer.
A piezoelectric transformer inverter is required to be controlled in such a manner that each value of an output voltage and an output current with respect to a load is set to be a desired value. As such a control system, a constant phase-difference control system is known. This system focuses on frequency characteristics of the phase difference between an input voltage and an output voltage of a piezoelectric transformer.
For example, Japanese Unexamined Patent Publication No. 9-237684 provides a system for controlling a pulse voltage signal driving a piezoelectric transformer, in which the phase difference between an input voltage and an output voltage of the piezoelectric transformer is detected to perform frequency control such that the phase difference value is set to be a specified value, and a tube current is detected to drive the piezoelectric transformer in such a manner that the tube current value is maintained to be a desired value.
Similarly, Japanese Unexamined Patent Publication No. 8-149850 provides a system in which the phase difference between an input voltage and an output voltage of the piezoelectric transformer is detected so as to perform frequency control in such a manner that the phase difference value is set to be a specified value. However, in the system described in Japanese Unexamined Patent Publication No. 9-237684, a pulse voltage obtained by controlling the duty ratio of a frequency higher than the driving frequency of the piezoelectric transformer is supplied to a piezoelectric transformer drive unit. In contrast, in the system described in Japanese Unexamined Patent Publication No. 8-149850, the piezoelectric transformer is intermittently driven by a frequency lower than the driving frequency of the piezoelectric transformer.
In addition, Japanese Unexamined Patent Publication No. 9-219292 describes a system in which the phase difference between an input voltage of a piezoelectric transformer and a detection voltage obtained by the voltage conversion of a tube current is detected to perform frequency control in such a manner that the phase difference value is set to be a desired value.
In addition, Japanese Unexamined Patent Publication No. 9-199289 describes a system in which the phase difference between an input voltage and an output voltage of a piezoelectric transformer is detected to perform frequency control in such a manner that the phase difference value is set to be a desired value. In both the aforementioned control systems described in Japanese Unexamined Patent Publication No. 9-219292 and Japanese Unexamined Patent Publication No. 9-199289, only the above-described frequency control is performed, and there is no description of a pulse-width control and burst control of a pulse voltage signal driving the piezoelectric transformer.
In the constant phase-difference control systems in the above-described conventional piezoelectric transformer inverters, one of the signals detecting the phase difference forms an input voltage waveform of the piezoelectric transformer. The input voltage waveform of the piezoelectric transformer is usually the waveform of a sine wave or is similar to that of a sine wave in order to efficiently drive the piezoelectric transformer.
The above sine wave is generated by a particular LC resonance. An input capacitance of the piezoelectric transformer is often used as the capacitance of a resonator.
FIG. 12 shows a typical equivalent circuit of a piezoelectric transformer. The equivalent input capacitance of the piezoelectric transformer changes greatly depending on various factors such as frequency, load, input voltage, and temperature.
FIG. 13 shows the frequency dependences of input capacitance and input impedance of a piezoelectric transformer. As is evident in FIG. 13, the input capacitance changes greatly according to the frequency. In addition to the frequency, as described above, the load, the input voltage, and the temperature permit the input capacitance to be greatly changed.
As a result, with the changes in the above-described parameters, distortion tends to occur in the input voltage waveform of the piezoelectric transformer. Furthermore, the cycle and the peak value of the input voltage waveform of the piezoelectric transformer change significantly.
Thus, it is very difficult to obtain a pure phase-difference detection signal from the input voltage waveform of the piezoelectric transformer.
In addition, when burst dimming is performed, it is necessary to intermittently operate the inverter at a frequency equivalent to or lower than the driving frequency of the piezoelectric transformer. In this case, since the input voltage waveform of the piezoelectric transformer does not occur during a burst-off period, it is impossible to obtain the phase-difference detection signal from the input voltage waveform of a piezoelectric transformer.
As described above, in Japanese Unexamined Patent Publication No. 9-219292 and Japanese Unexamined Patent Publication No. 9-199289, the frequency control is performed in such a manner that the phase difference is set to be constant. However, since the phase difference is approximately depended on frequency, when the phase difference is controlled to be constant, the frequency also becomes constant. Thus, when the input voltage has a wide range, for example, from 7 to 20V, tube current increases or decreases approximately in proportion to the input voltage. As a result, since the luminance of a fluorescence tube is approximately proportional to the tube current, the luminance becomes unstable due to the changes in the input voltage.
In addition, in Japanese Unexamined Patent Publication No. 9-237684, with respect to changes in input voltage, the input voltage of the piezoelectric transformer is stabilized by adjusting the electrical power of an oscillation pulse voltage signal outputted at a part where the piezoelectric transformer is driven. However, dimming is performed by continuously controlling the tube current by controlling an average input voltage of the piezoelectric transformer. As a result, when a widely ranging dimming such as a dimming ranging from 10 to 100% is performed, the above-described control systems cannot be used. In other words, the normal range of the tube current of a cold-cathode tube is narrow, for example, ranging from 2 to 5 mA rms, and the luminance is approximately proportional to the tube current. As a result, it is very difficult to broaden the dimming range by using a dimming method which continuously controls the tube current.
Furthermore, in Japanese Unexamined Patent Publication No. 8-149850, the piezoelectric transformer is intermittently driven by a frequency lower than the piezoelectric transformer driving frequency, by which the tube current is switched on and off by a frequency equivalent to or higher than a visible frequency so as to achieve dimming. For example, when an on and off ratio is set to be in a range of 10% to 100%, a dimming range of 10% to 100% is obtained.
However, in a manner similar to that of the aforementioned cases, in the conventional art described in Japanese Unexamined Patent Publication No. 8-149850, when the input voltage has a wide range, such as a range of 7 to 20V, the tube current obtained when the tube current is in an on state increases and decreases approximately in proportion to the input voltage. Therefore, the luminance becomes unstable due to changes in the input voltage.